Electrical reactor

ABSTRACT

A shunt reactor, including at least two opposing winding sections separated by magnetic shielding material. Saturation of the magnetic shielding material at a predetermined voltage produces a large drop in the reactance of the reactor, due to interaction of the opposing fluxes of the winding sections.

United States Patent Saul Bennon Muncie, 1nd.

App]. No. 89,398

Filed Nov. 13, 1970 Patented Nov. 16, 197 l Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

inventor ELECTRICAL REACTOR 16 Claims, 6 Drawing Figs.

U.S.'Cl 336/84,

Int. Cl 11011 15/04 Field of Search 336/84, 90, 92, 94,170,180, 212, 214, 215

{56] References Cited UNITED STATES PATENTS 1,779,269 10/1930 Clough 336/215 X 3,160,839 12/1964 Bennon et al 336/84 Primary Examiner-Thomas J. Kozma Attorneys-A. T. Stratton, F. E. Browder and D. R. Lackey ABSTRACT: A shunt reactor, including at least two opposing winding sections separated by magnetic shielding material. Saturation of the magnetic shielding material at a predetermined voltage produces a large drop in the reactance of the reactor, due to interaction of the opposing fluxes of the winding sections.

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ELECTRICAL REACTOR BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates in general to electrical reactors, and more specifically to shunt reactors of the liquid filled, air-core type.

2. Description of the Prior Art Long high-voltage (HV) and extra high-voltage (El-IV) transmission lines, with high voltage being defined as 100 kv. to 229 kv., and extra high voltage being over 230 kv., produces certain reactive volt-ampere (VAR) requirements on the end systems connected to the transmission lines. If the end systems are unable to absorb the VARS with inductive loads, terminal voltages may rise to magnitudes which may damage apparatus connected thereto. The VAR requirements for transmission lines increase with the square of the voltage, and are a function of the line capacitance and length. The increased capacitance of bundled conductors commonly used for EHV transmission lines, has greatly increased the VAR requirements, compared with the conductors normally used with the high-voltage transmission lines.

Thus, it is common to provide compensation for long HV and EHV transmission lines which may have periods of light loads, or transmission lines which are lightly loaded in the early stages of their development, by connecting shunt reactors to the HV or EHV line at the receiving end of the system; and, depending upon the line length and voltage profile desired across the transmission line, they may also be connected to the line at one or more selected intermediate points.

A shunt reactor may be directly connected to the transmission line, to a tertiary winding of a step-up or stepdown transformer, or to the lower voltage bus to which the transformer is connected. If the shunt reactor is connected to the lower voltage bus, the charging reactive power must be transformed to the transmission line through the associated transformer. This reactive power flow creates a rise in voltage through the transformer leakage reactance, and during light load conditions, it may be necessary to reduce the magnitude of thelow voltage, which results in reducing the amount of the compensation. Connecting the shunt reactor to the tertiary winding of the step-up or stepdown transformer reduces the voltage variation and thus more effectively utilizes the reactor, but the required transformation of large blocks of reactive power can result in an increase in the tertiary rating over the standard rating. If the impedance of the tertiary winding is such that there is a significant drop in tertiary voltage as shunt reactive volt-amperes are increased, it may be necessary to use nominally higher rated reactors to achieve the required compensation, since the KVAR varies directly as the square of the reactor terminal voltage.

Thus, it would be desirable to be able to connect at least a base value of inductive reactance directly to the transmission line, i.e., that amount which does not have to be switched as the load changes. When shunt reactors are connected to a transmission line intermediate its ends, a transformer is not available, nor is the lower voltage bus, and by necessity the shunt reactor must be designed for operation at the transmission line voltage.

There are two main types of shunt reactors, the air core, and the iron core. In the usual air-core reactor, the magnetic flux field is not shaped, but is determined by the proportions of the winding. This type of reactor includes a cable-wound coil with suitable mechanical bracing, and solid insulation to ground them between turns, and since it is open to the atmosphere,

' i.e., air cooled, it is limited to the lower voltage applications,

i.e., to applications where the shunt reactor is connected to the tertiary winding of the transformer, or to the lower voltage bus connected to the transformer.

The iron-core shunt reactor has a plurality of airgaps in the iron circuit, in order to drastically reduce the amount of iron which would otherwise be required, and to achieve a more nearly sinusoidal load current. The airgaps, however, produce design problems due to flux fringing at the gaps, and possible high noise level due to vibration. When the iron portion of the magnetic circuit saturates due to overvoltage, the flux field reverts to the air-core mode of operation during the portion of the voltage cycle that saturation takes place, dropping the reactors inductance to about 25 percent of its normal value. This drastic and rapid change in the inductance of the shunt reactor may cause undesirable system transients, but it does have an advantage in very lightly loaded transmission lines in being able to drag dangerous overvoltages down by drawing large currents through the reactor during saturation.

U.S. Pat. No. 3,160,839, which is assigned to the same assignee as the present application, discloses a shunt reactor which is basically of the air-core construction, but which enables an air-core reactor to be directly connected to the transmission line by using oil, and oil-impregnated solid insulation, to insulate the reactor. The metallic tank which must be used to hold the oil is shielded from the reactor winding by magnetic shields which separate the winding from the tank walls and tank bottom. Thus, it is essentially an air-core reactor, of instead 0 air disposed within the coils, the coils are filled with nonmagnetic material, i.e., oil and solid insulation. Except for the lack of magnetic material within the coils or winding, the construction is similar to that of shell-form power transformers, and it thus has the advantages of this type of transformer, such as being able to distribute surge potentials substantially uniformly across the serially connected pancake coils, and to minimize the voltage oscillations which are produced when a capacitive distribution of voltage changes to inductive. It also eliminates the flux fringing and vibration problems of the gapped iron-core reactor. This type of reactor construction also provides a more linear impedance characteristic, even when the shield circuit saturates for a portion of each voltage cycle, dropping to an ohmic impedance of about 70 percent of the unsaturated shield circuit value, providing a more nearly sinusoidal current during overvoltage conditions than a gapped iron-core reactor.

While the smaller inductance change of the liquidfilled, shielded air-core type reactor upon saturation of the shield circuit, compared with the inductance change of the gapped iron-core reactor upon saturation of the magnetic portions of its magnetic circuit, is an advantage in most transmission line systems, there are certain instances when it would be desirable to reduce the ohmic impedance of the reactor, when its magnetic shield circuit is saturated, to about 50 percent of its unsaturated value. A shunt reactor with this characteristic would be more effective in reducing the magnitude of voltage peaks than one with a higher saturated impedance, as it would draw more current during periods of saturation and thus reduce the voltage peaks accordingly. However, it would be desirable to be able to achieve this characteristic while utilizing the liquidfilled, shielded, air-core construction disclosed in the hereinbefore mentioned U.S. patent.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved shunt reactor constructed to reduce its effective inductance more drastically upon reaching a predetermined overvoltage, compared with the construction disclosed in U.S. Pat. No. 3,160,839, while obtaining the constructional and operational advantages of the shunt reactor disclosed in this patent. The reactor is of the liquid-filled, shielded, air-core type, having a plurality of pancake coils disposed in a tank filled wit insulating and cooling oil, with the pancake coils being oriented such that their central openings are substantially aligned. Nonmagnetic means is disposed in the coil openings. Magnetic shielding members are disposed between the pancake coils and the sidewall and bottom portions of the tank, and also between predetermined adjacent pancake coils, to provide at least first and second coil sections or groups, with each group being surrounded by magnetic material. The pancake coils are wound and interconnected such that the axial directions of the mag netic flux produced by adjacent coil groups are opposite, i.e., in opposition to one another. The shunt reactor, when unsaturated, operates with a normal linear characteristic, without interaction of these oppositely directed fluxes, because each coil group has its own complete magnetic circuit which surrounds it. Upon saturation of the shielding member which separates adjacent coil groups, the fluxes of each coil group no longer have the low reluctance closed magnetic loops about these coil groups, and they are allowed to interact, oppose, and cancel, providing the desired substantial change in the effective inductance of the reactor.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood, and further advantages and uses thereof more readily apparent, when considered in view of the following detailed description of exemplary embodiments thereof, taken with the accompanying drawings, in which:

FIG. 1 is a plan view of a shunt reactor constructed according to the teachings of the invention;

FIG. 1A is a schematic diagram of the shunt reactor shown in FIG. 1; V

FIG. 2 is a perspective view partially cut away reactor partially in section, of the shunt reactor shown in FIG. 1;

FIG. 3 is a plan view of a shunt reactor constructed according to another embodiment of the invention;

FIG. 3A is a schematic diagram of the shunt reactor shown in FIG. 3; and,

FIG. 3B is a schematic diagram illustrating an alternate winding arrangement which may be used.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIGS. 1 and 2 in particular, there are shown plan and perspective views, respectively, of a shunt reactor constructed according to a first embodiment of the invention. Reactor 10 includes a singlephase winding assembly 12 disposed in a metallic tank or casing 14, having sidewall and bottom portions and 7, respectively, with the tank 14 being filled to a level 9 with a liquid insulating and cooling dielectric, such as mineral oil. The tank cover and insulating bushings are not shown in order to simplify the drawings. Winding assembly 12 is immersed in the liquid dielectric, which aids in insulating the winding for the transmission line voltage at which the r reactor is designed to accommodate, as well as to remove heat from the winding by circulating through cooling ducts therein, and through exterv nal heat exchanges (not shown).

Winding 12 includes a plurality of pancake-type coils, such as pancake coils 16, l7, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, and 31, which have a substantially rectangular outer configuration, first and second substantially flat, major opposed sides or surfaces, and a substantially rectangular central opening. Only a sufficient number of pancakes are shown to adequately describe the invention, with the number used being determined by the specific application. The pancake coils of winding 12 are disposed in tank 14 in spaced, side-byside relation, with their central openings substantially aligned on a common axis 33. The pancake coils are oriented to provide vertical ducts or spaces between them, for the circulation of the liquid dielectric. The cooling ducts are provided, as illustrated in FIG. 2, by insulating spacer washer member, such as washer member 32, which have a plurality of spaced insulating blocks 34 attached thereto to create a tortuous path for the liquid dielectric adjacent to the major outer surfaces of the pancake coils.

Reactor their is of the air-core type, as the pancake coils do not have magnetic material disposed through their central openings. Instead of air being disposed in their central openings, however, their openings are filled with nonmagnetic means 36, in the form of the liquid dielectric utilized, and solid insulating members. The solid insulating members provide support for the coils, and they brace the coils against movement during certain operating conditions of the reactor.

The sidewall portions 15 of the tank 14, are generally formed of magnetic steel, and are shielded from the winding 12 by shielding means 40 disposed such that certain of its members are between the winding 12 and the sidewall portions, with the shielding means 40 providing a low reluctance path for the magnetic flux produced by the winding, 12, shunting the flux away from the tank walls to prevent excessive heating and losses in the tank walls due to eddy currents and hysteresis, which would otherwise occur. Shielding means 40 includes a plurality of stacked layers 42 of metallic, magnetic laminations, such as cold-rolled silicon steel, which have a preferred direction of magnetic orientation substantially parallel with their longitudinal dimensions. The stacked layers provide shielding members adjacent each of the sidewall portions of the tank 14. Each layer of laminations includes laminations 44, 46, 48, and 50, disposed adjacent to the sidewall portions 15, with their major surfaces perpendicular thereto, and these laminations are assembled to form a closed magnetic loop. The ends of the laminations may be mitered, as shown, to provide joints in the loop which have a low reluctance to the magnetic flux, and the diagonal corner joints may be offset from one another from layer to layer in any desired arrangement, such as the butt-lap arrangement, to further reduce the reluctance of the shielding means 40. Shielding means 40 has its members spaced from the tank walls by a plurality of vertically oriented, spaced insulating wedge or spacer members 41, which may be formed of hardwood, which members aid in bracing the shielding means during certain operating conditions of the reactor, as well as preventing the tank walls from shorting the laminations. These bracing members also provide vertical cooling ducts for circulation of the liquid dielectric adjacent to the outer surfaces of the shielding members of the shielding means 40.

Instead of disposing the complete single-phase winding assembly 12 in a single window or opening of the shielding means 40, as taught by U.S. Pat. No. 3,160,839, the singlephase winding assembly 12 is divided into two or more axially spaced coil sections or groups, each disposed in a separate or individual window of the shielding means 40, with the coil groups being wound and interconnected such that the axial directions of the magnetic flux produced by adjacent coil groups are opposite at any instant.

In the embodiment of the invention shown in FIGS. 1 and 2, the singlephase winding assembly is divided into first, second, and third axially spaced coil groups 52, 54 and 56 respectively, which are disposed in openings or windows 58, 60, and 62, respectively, defined by the shielding means 40. The windows 58, 60, and 62 are provided by adding two laminations 64 and 66 to each layer 42 of laminations, with the longitudinal center lines of laminations 64 and 66 being spaced from and parallel with the longitudinal center lines of the outer laminations 44 and 48. Laminations 64 and 66 may have their ends cut diagonally, which ends match similar V-cuts in the edges of laminations 46 and S0, to provide low-reluctance joints between these additional laminations and the outer laminations.

In this embodiment, a high-voltage terminal 68 is connected to the midpoint of the second coil group 54, between pancake coils l6 and 17, and the circuit proceeds in both axial directions, with the first axial direction including pancake coils 16, 18, 20, 22, 24, and 26 of the second coil group, and pancake coils 28 and 30 of the third coil group 56, and with the second axial direction including pancake coils 17, 19, 21, 23, 25, and 27 of the second coil group, and pancake coils 29 and 31 of the first coil group 52. Adjacent pancake coils of the second coil group 54 are interconnected with start-start, finish-finish connections, with the high-voltage terminal 68 being connected to the finish" or end of the outermost turn of pancake coil 16. Pancake coil 16 has its innermost turn connected to the innermost turn of pancake coil 18 via'startstart connection 70, pancake coils l8 and 20 are interconnected with finish-finish connection 72, pancake coils 20 and 22 are interconnected with start-start connection 74, pancake coils 22 and 24 are interconnected with finish-finish connection 76, and pancake coils 24 and 26 are interconnected with start-start connection 78. The pancake coils l6, 18, 20, 22, 24, and 26 of the second coil section are wound and interconnected to provide an additive magnetomotive force which produces a magnetic field having a predetermined axial direction through the openings of the pancake coils at any instant. The pancake coils 28 and 30 of the third coil group are wound and interconnected, and the second and third coils groups are interconnected such that the axial direction of the magnetic flux or magnetic lines of force of the third coil group 56 is opposite to the axial direction of the magnetic flux produced by the second coil group 54 at any instant. For example, instead of connecting the pancake coil 26 of the second coil group 54 to the nearest pancake coil of the third coil group, the pancake coils 28 and 30 may be wound such that the desired flux opposition may be obtained by connecting the finish or outer turn of pancake coil 26 to the finish or outer turn of pancake coil 30, via conductor 80. Pancake coils 30 and 28 are interconnected with start-start connection 82, and the finish of pancake coil 28 is connected to a neutral terminal, or to ground 84, as required by the specific application. However, it is to be understood that pancake coils 28 and 30 may be wound such that the interconnection between the second and third coil groups may be made between the outermost turns of pancake coils 26 and 28, ans still provide the opposition required between their magnetic fluxes.

The high-voltage terminal 68 is connected to the finish or outermost turn of pancake coil 17, and pancake coils 17, 19,

21, 23, 25, and 27 are serially connected with start-start, finish-finish connections, with pancake coils l7 and 19 being interconnected with start-start connection 86, pancake coils l9 and 21 being interconnected with finish-finish connection 88, pancake coils 21 and 23 being interconnected with startstart connection 90, pancake coils 23 and 25 being interconnected with finish-finish connection 92, and pancake coils 25 and 27 being interconnected with start-start connection 94. Pancake coils l7, 19, 21, 23, 25, and 27 are wound and interconnected such that the magnetic flux produced by current flow therethrough adds to the magnetic flux produced by the other pancake coils of the second coil section 54. The pancake coils 29 and 31 disposed in the first opening 58 of the shielding means 40, are wound and interconnected with the second coil group such that the magnetic flux produced by the pancake coils of the first coil section 52 opposes the magnetic flux produced by the second coil section 54, at any instant. For example, the outermost turn of pancake coil 27 of the second coil group may be connected to the outermost turn of pancake coil 31, pancake coils 31 and 29 may be connected together via start-start connection 98, and the end of the outermost turn of pancake coil 29 may be connected to a neutral terminal, or to ground 100, as required. The magnetic shielding of winding 12 is completed by shielding the bottom portion 7 of the tank 14 from the magnetic flux produced by winding 12, with the shielding of the bottom portion 7 being accomplished by disposing a plurality of bundles 85 of stacked metallic laminations adjacent to the bottom, and spaced therefrom by suitable nonmagnetic members, such as members formed of wood, with the laminations of the bundles having their major surfaces disposed perpendicular to the bottom portion 17. It will usually not be necessary to shield the top portion (not shown) of the tank 14 from the winding 12, as it is spaced therefrom by a distance sufficient to prevent excessive heating and losses in the top or cover portion of the tank.

Since the high-voltage terminal 68 is connected to the mid point of the second coil section 54, the pancake coils may be graded dimensionally to provide clearances between the pancake coils and the grounded shielding means 40, according to the magnitudes of their electrical potentials. Further, the pancake coils may be slightly dished to grade the clearance between adjacent pancake coils according to the potential between them at adjacent radial locations.

FIG. 1A is a schematic diagram of the winding arrangement shown in FIGS. 1 and 2, illustrating the two parallel circuits between the high-voltage terminal 68 and ground, or to a neutral terminal. The arrows indicate the directions of the axial magnetic fields associated with each coil group, at a selected time instant.

In the operation of the shunt reactor 10, high-voltage terminal 68 would be connected directly to a transmission line at a predetermined point, or it could be connected to the tertiary winding of a step-up or stepdown transformer, if available, depending upon the design requirements, and the ends of the first and third coil groups would be grounded or connected to a neutral bushing of a three-phase shunt reactor arrangement, as required. Each coil group has a closed magnetic loop surrounding it. Thus, even though the instantaneous axial directions of the magnetic fluxes of adjacent coil groups are opposite to one another, there is substantially no interaction of the oppositely directed fluxes as they each follow the lowreluctance circuit provided by the closed magnetic loop associated with its coil group. This is illustrated in FIG. I, with the solid arrows indicating the flux from the second coil group, and the broken arrows indicating the flux circulating about the magnetic loops of the first and third coil groups. Thus, when the magnetic loops are not saturated, the operation of a reactor 10 is similar to that of the reactor disclosed in U.S. Pat. No. 3,160,839. Upon a predetermined overvoltage condition, however, the magnetic shielding means 64 and 66 disposed between the adjacent first and second, and second and third coil groups, respectively, saturates, with the width of these laminations being selected to initiate saturation at the desired overvoltage magnitude. When the magnetic shielding means disposed between the coil groups or sections saturates, the reactance of reactor 10 will drop substantially, due to the interaction of the opposing fluxes. The outer portion of the shielding means, including the members formed of laminations 44, 46, 48, and 50, may be designed to saturate at the same overvoltage magnitude as the shielding means disposed between adjacent coil sections, to provide the maximum change in reactance in one step at a predetermined voltage magnitude, or this portion of the magnetic circuit may be designed to saturate at a different voltage level, to change the reactance of the reactor 10 in steps, as desired. The number of pancake coils in the opposing end sections is also selected to provide the desired interaction and cancellation of fluxes at saturation, and thus control the magnitude of the reactance change.

FIG. 3 is a plan view of a shunt reactor 10' constructed according to another embodiment of the invention, with like reference numerals in FIGS. 1 and 3 indicating like components. Like reference numerals, except for a prime mark, indicate similar but modified components. Reactor 10 is essentially one-half reactor 10 shown in FIG. 1, having a single series circuit, instead of two series circuits connected in parallel, between a source of high voltage and a neutral or grounded terminal. Reactor 10' has a magnetic shielding circuit 40 disposed between the sidewall portions 15' of a metallic tank 14', and a plurality of pancake coils l6, 18, 20, 22, 24, 26, 28, and 30, which make up a single-phase winding assembly 12'. The magnetic shielding means 40' has a plurality of stacked layers of metallic, magnetic laminations, with each layer including laminations 44', 46', 48, 50' and 66. Laminations 44, 46', 48, and 50' are assembled with their ends butting together with mitered joints, and lamination 66 is disposed with its longitudinal dimension parallel to the longitudinal dimensions of laminations 44' and 48, to provide windows or openings 60 and 62. Pancake coils l6, 18, 20, 22, 24, and 26 are disposed in window 60', and pancake coils 28 and 30 are disposed in window 62.

The pancake coils of the first sections are interconnected with start-start, finish-finish connections, with the high-voltage terminal 68 being connected to the outermost turn of pancake coil 16. Pancake coils 16 and 18 are interconnected with start-start connection 70, pancake coils l8 and 20 are interconnected with finish-finish connection 72, pancake coils 20 and 22 are interconnected wit start-start connection 74, pancake coils 22 and 24 are interconnected with finish-finish connection 76, and pancake coils 24 and 26 are interconnected with start-start connection 78. All of the pancake coils of the first section are wound and interconnected to provide an additive magnetic flux in a predetermined axial direction through the openings of the pancake coils, and the pancake coils of this section are interconnected with the pancake coils 28 and 30 of the second section such that the magnetic flux produced by the second section has an axial direction opposite to the axial direction of the flux of the first section, at any given instant. For example, as illustrated, the outermost turn of pancake coil 26 of the first group may be connected to the outermost turn of pancake coil 30 of the second group, via finishfinish connection 80, pancake coils W8 and 30 may be interconnected with start-start connection 82, and the end of the outermost turn of pancake coil 28 may be connected to ground 84, as illustrated, or to a neutral terminal, as required.

In the operation of the reactor shown in FIG. 3, there is substantially no interaction between the oppositely directed magnetic fluxes as long as the member 66 disposed between the coil groups is unsaturated. Once a predetermined overvoltage condition is reached which saturates shielding member 66, the fluxes are no longer directed through the lowreluctance magnetic loops about each coil group, but now are directly opposed, cancelling one another, an sharply reducing the effective inductance or reactance of the reactor 10'. The outermost portion of the shielding means 40' may be dimensioned to saturate at the same voltage magnitude as the interposed shielding portion 66, or it may be designed to, saturate at a different voltage, as desired to obtain the required change in inductance of the reactor 10'.

FIG. 3A is a schematic diagram of the connection of the reactor 10', with the arrows associated with the winding sections illustrating the opposite axial directions of the magnetic flux produced by the coil sections at any given instance.

FIG. 3B is a schematic diagram which illustrates that the opposing winding sections may be connected in parallel as well as in series. Like reference numerals in FIGS. 3, 3A and 3B indicate like components with double prime marks added in the embodiment of FIG. 3B.

In summary, there has been disclosed a new and improved shunt reactor which may be connected directly to a high-voltage, or extra high-voltage transmission line, and which utilizes the constructional and operational advantages of the reactor disclosed in US. Pat. No. 3,160,839, while obtaining a greater change in reactance at a predetermined overvoltage condition, than the reactor disclosed in the patent. The greater change in reactance is an advantage in certain applications, such as very lightly loaded, long, high-voltage or extra highvoltage transmission lines. The disclosed construction does not have the flux fringing or noise problems associated with gapped iron-core magnetic circuit reactors, but it possess the advantage of this type reactor in producing a substantial ,change in its inductive reactance at a predetermined overvoltage condition of the transmission line, to draw more current and thus load the transmission line more heavily during the overvoltage condition. For example, the ohmic impedance of the disclosed shunt reactor, when the shield circuit is saturated, may be selected to be about 40 to 50 percent of the unsaturated value, compared with about 70 percent for the prior art shielded, liquid-filled air-core reactor. This value provides the desired regulating effect on the transmission line voltage, without introducing transients which a still more drastic change in reactance may create.

1 claim as my invention:

1. A reactor, comprising:

a metallic casing having sidewall and bottom portions,

a liquid dielectric disposed in said casing,

a plurality of pancake coils having central openings therein,

said plurality of pancake coils being disposed in said casing with their central openings substantially aligned,

magnetic shielding means disposed in said casing, between the sidewall portions of said casing and said plurality of pancake coils, and between certain adjacent pancake coils, to divide the plurality of pancake coils into at least first and second coil groups, each surrounded by said magnetic shielding means,

nonmagnetic means disposed in the central openings of said plurality of pancake coils, with said central openings being devoid of magnetic core means,

and means connecting said plurality of pancake coils to provide a single-phase winding assembly, wherein the instantaneous axial directions of the magnetic flux in the first and second coil groups are in opposition, to provide a substantial to in the reactance of the reactor when the magnetic shielding means disposed between the coil groups saturates.

2. The reactor of claim 1 wherein the magnetic shielding means divides the plurality of pancake coils into first, second and third coil groups, each surrounded by the magnetic shielding means, and the means which connects the pancake coils into a single-phase winding assembly directs the instantaneous magnetic flux of any two adjacent coil groups in opposite axial directions.

3. The reactor of claim 2 wherein the second coil group is disposed between the first and third groups, and including a high-voltage terminal connected to the midpoint of the second coil group.

4. The reactor of claim 1 including magnetic shielding means disposed between the plurality of pancake coils and the bottom portion of the casing.

5. The reactor of claim 1 wherein the shielding means disposed between the pancake coils and the sidewall portions of the casing is dimensioned to saturate at a different voltage magnitude than the shielding means disposed between the certain adjacent pancake coils.

6. The reactor of claim 1 wherein the shielding means disposed between the pancake coils an the sidewall portions of the casing is dimensioned to saturate at substantially the same voltage as the shielding means disposed between the certain adjacent pancake coils.

7. A reactor, comprising:

a metallic casing having sidewall and bottom portions,

a liquid dielectric disposed in said casing,

magnetic shielding means disposed in said casing, including a plurality of stacked layers of metallic laminations assembled to form a plurality of substantially rectangular windows, with a closed magnetic loop being provided about each window,

a plurality of pancake coils each having a central opening therein,

said plurality of pancake coils being divided into groups,

with each group being disposed in a different window of said magnetic shielding means, said plurality of pancake coils being oriented such that their central openings are substantially aligned,

nonmagnetic means disposed in the central openings of said plurality of pancake coils, with said central openings being devoid of magnetic core means,

and means connecting said plurality of pancake coils to provide a single-phase winding assembly, wherein the instantaneous axial directions of the magnetic flux of any two adjacent coil groups are opposite to one another, to provide a substantial change in the inductive reactance of the reactor when the magnetic shielding means disposed between two adjacent coil groups saturates.

8. The reactor of claim 7 wherein the magnetic shielding means defines first and second rectangular windows, and the plurality of pancake coils are divided into first and second groups, which groups are disposed in said first and second windows, respectively.

9. The reactor of claim 7 wherein the magnetic shielding means defines first, second and third rectangular windows, and the plurality of pancake coils are divided into first, second, and third groups, which groups are disposed in said first, second, and third windows, respectively.

10. The reactor of claim 7 including magnetic shielding means disposed between the bottom portion of the casing and the plurality of pancake coils.

11. The reactor of claim 7 wherein the magnetic shielding means defines first, second, and third rectangular windows, with the second window being disposed between said first and third windows, and the plurality of pancake coils are divided into first, second, and third groups, each having first and second ends, which groups are disposed in said first, second, and third windows, respectively, and including a high-voltage terminal connected to the midpoint of said second group, the first end of said second group being connected to the first end of said first group, and the second end of said second group being connected to the second end of said third group.

12. The reactor of claim 7 wherein the shielding means disposed between the pancake coils and the sidewall portions of the casing is dimensioned to saturate at a different voltage magnitude than the shielding means disposed between the certain adjacent pancake coils.

13. The reactor of claim 7 wherein the shielding means disposed between the pancake coils and the sidewall portions of the casing is dimensioned to saturate at substantially the same voltage as the shielding means disposed between the certain adjacent pancake coils.

14. A reactor, comprising:

a casing,

magnetic shielding means disposed in said casing, said mag netic shielding means being arranged to provide at least first and second windows having a closed magnetic loop about each window, and a portion common to each magnetic loop,

first and second winding means having central openings disposed in the first and second windows, respectively, of said magnetic shielding means, with the central axes of said first and second winding means being coaxial, nonmagnetic means disposed in the central openings of said first and second winding means, with said central openings being devoid of magnetic core means,

terminal means adapted for connection to a source of single-phase alternating potential,

means connecting said first and second winding means and said terminal means to direct the magnetic flux provided by said first and second winding means in opposite axial directions at any instant,

the dimensions of the portion of said shielding means common to both magnetic loops being selected to saturate when a predetermined voltage magnitude is applied to said terminal means, providing a substantial change in the inductive reactance of the reactor at said predetermined voltage magnitude.

15. The reactor of claim 14 wherein the first and second winding means are serially connected with respect to the terminal means.

16. The reactor of claim 14 wherein the first and second winding means are connected in parallel with respect to the terminal means.

l l 1k 

1. A reactor, comprising: a metallic casing having sidewall and bottom portions, a liquid dielectric disposed in said casing, a plurality of pancake coils having central openings therein, said plurality of pancake coils being disposed in said casing with their central openings substantially aligned, magnetic shielding means disposed in said casing, between the sidewall portions of said casing and said plurality of pancake coils, and between certain adjacent pancake coils, to divide the plurality of pancake coils into at least first and second coil groups, each surrounded by said magnetic shielding means, nonmagnetic means disposed in the central openings of said plurality of pancake coils, with said central openings being devoid of magnetic core means, and means connecting said plurality of pancake coils to provide a single-phase winding assembly, wherein the instantaneous axial directions of the magnetic flux in the first and second coil groups are in opposition, to provide a substantial to in the reactance of the reactor when the magnetic shielding means disposed between the coil groups saturates.
 2. The reactor of claim 1 wherein the magnetic shielding means divides the plurality of pancake coils into first, second and third coil groups, each surrounded by the magnetic shielding means, and the means which connects the pancake coils into a single-phase winding assembly directs the instantaneous magnetic flux of any two adjacent coil groups in opposite axial directions.
 3. The reactor of claim 2 wherein the second coil group is disposed between the first and third groups, and including a high-voltage terminal connected to the midpoint of the second coil group.
 4. The reactor of claim 1 including magnetic shielding means disposed between the plurality of pancake coils and the bottom portion of the casing.
 5. The reactor of claim 1 wherein the shielding means disposed between the pancake coils and the sidewall portions of the casing is dimensioned to saturate at a different voltage magnitude than the shielding means disposed between the certain adjacent pancake coils.
 6. The reactor of claim 1 wherein the shielding means disposed between the pancake coils an the sidewall portions of the casing is dimensioned to saturate at substantially the same voltage as the shielding means disposed between the certain adjacent pancake coils.
 7. A reactor, comprising: a metallic casing having sidewall and bottom portions, a liquid dielectric disposed in said casing, magnetic shielding means disposed in said casing, including a plurality of stacked layers of metallic laminations assembled to form a plurality of substantially rectangular windows, with a closed magnetic loop being provided about each window, a plurality of pancake coils each having a central opening therein, said plurality of pancake coils being divided into groups, with each group being disposed in a different window of said magnetic shielding means, said plurality of pancake coils being oriented such that their central openings are substantially aligned, nonmagnetic means disposed in the central openings of said plurality of pancake coils, with said central openings being devoid of magnetic core means, and means connecting said plurality of pancake coils to provide a single-phase winding assembly, wherein the instantaneous axial directions of the magnetic flux of any two adjacent coil groups are opposite to one another, to provide a substantial change in the inductive reactance of the reactor when the magnetic shielding means disposed between two adjacent coil groups saturates.
 8. The reactor of claim 7 wherein the magnetic shielding means defines first and second rectangular windows, and the plurality of pancake coils are divided into first and second groups, which groups are disposed in said first and second windows, respectively.
 9. The reactor of claim 7 wherein the magnetic shielding means defines first, second and third rectangular windows, and the plurality of pancake coils are divided into first, second, and third groups, which groups are disposed in said first, second, and third windows, respectively.
 10. The reactor of claim 7 including magnetic shielding means disposed between the bottom portion of the casing and the plurality of pancake coils.
 11. The reactor of claim 7 wherein the magnetic shielding means defines first, second, and third rectangular windows, with the second window being disposed between said first and third windows, and the plurality of pancake coils are divided into first, second, and third groups, each having first and second ends, which groups are disposed in said first, second, and third windows, respectively, and including a high-voltage terminal connected to the midpoint of said second group, the first end of said second group being connected to the first end of said first group, and the second end of said second group being connected to the second end of said third group.
 12. The reactor of claim 7 wherein the shielding means disposed between the pancake coils and the sidewall portions of the casing is dimensioned to saturate at a different voltage magnitude than the shielding means disposed between the certain adjacent pancake coils.
 13. The reactor of claim 7 wherein the shielding means disposed between the pancake coils and the sidewall portions of the casing is dimensioned to saturate at substantially the same voltage as the shielding means disposed between the certain adjacent pancake coils.
 14. A reactor, comprising: a casing, magnetic shielding means disposed in said casing, said magnetic shielding means being arranged to provide at least first and second windows having a closed magnetic loop about each window, and a portion common to each magnetic loop, first and seconD winding means having central openings disposed in the first and second windows, respectively, of said magnetic shielding means, with the central axes of said first and second winding means being coaxial, nonmagnetic means disposed in the central openings of said first and second winding means, with said central openings being devoid of magnetic core means, terminal means adapted for connection to a source of single-phase alternating potential, means connecting said first and second winding means and said terminal means to direct the magnetic flux provided by said first and second winding means in opposite axial directions at any instant, the dimensions of the portion of said shielding means common to both magnetic loops being selected to saturate when a predetermined voltage magnitude is applied to said terminal means, providing a substantial change in the inductive reactance of the reactor at said predetermined voltage magnitude.
 15. The reactor of claim 14 wherein the first and second winding means are serially connected with respect to the terminal means.
 16. The reactor of claim 14 wherein the first and second winding means are connected in parallel with respect to the terminal means. 